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Green tea

Observational studies suggest that regular consumption of coffee or tea may decrease the risk of developing type 2 diabetes. But what effect does coffee or tea have on blood glucose levels, glycated haemoglobin (HbA1c) and insulin?

Kondo and colleagues recently conducted a systematic review and meta-analysis of randomised controlled trials, specifically looking at the effect of coffee and tea consumption on fasting blood glucose levels, HbA1c and insulin in a range of men and women, including those who were healthy, overweight, had pre-diabetes, type 2 diabetes or the metabolic syndrome.

A total of 27 studies met the authors inclusion criteria, and included nearly 1,900 people living in Asia, Europe and the USA.

Coffee and black tea had no significant effect on fasting blood glucose (FBG), however, green tea slightly reduced FBG levels by 2.1 mg/dL (0.12 mmol/L). Interestingly, the effect was stronger (2.51 mg/dL (0.14 mmol/L)) for younger people (aged less than 55 years) and those living in Asian countries (3.81 mg/dL (0.21 mmol/L)).

Neither coffee nor tea had any significant effects on glycated haemoglobin, which is a measure of longer-term (three month) average blood glucose levels.

Coffee slightly increased fasting blood insulin levels (1.1 uIU/mL), but neither black nor green tea had any effect. Neither coffee nor tea of any kind had any effect on Homeostatic Model Assessment for Insulin Resistance (HOMA-IR), which is a measure of insulin sensitivity and pancreatic β-cell function.


Coffee and black tea appear to have no effect on blood glucose or insulin levels, however, green tea may slightly lower fasting blood glucose, particularly in younger people living in Asia where green tea consumption is more common.

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Dr Alan Barclay, PhD, is a consultant dietitian and chef with a particular interest in carbohydrates and diabetes. He is author of Reversing Diabetes (Murdoch Books), and co-author of 40 scientific publications, The Good Carbs Cookbook (Murdoch Books), Managing Type 2 Diabetes (Hachette Australia) and The Ultimate Guide to Sugars and Sweeteners (The Experiment Publishing).
Contact: Follow him on Twitter, LinkedIn or check out his website.